1. Field of the Invention
This invention relates to a process for decomposing pollutants (in particular, organochlorine compounds) and a pollutant decomposition system used therefor.
2. Related Background Art
With the development of industrial techniques until recent years, the use of organochlorine compounds such as ethylene chloride and methane chloride has been widespread. Disposal of these compounds has become a serious concern. Due to environmental problems caused by these pollutants, great efforts are being made to remediate these problems.
As methods for disposing of such pollutants, for example, methods are available in which ethylene chloride is decomposed with an oxidant or a catalyst. Stated specifically, known are a method in which it is decomposed with ozone (Japanese Patent Application Laid-Open No. 3-38297) and a method in which it is irradiated by ultraviolet rays in the presence of hydrogen peroxide (Japanese Patent Application Laid-Open No. 63-218293). It is also suggested to use sodium hypochlorite as an oxidizing agent (U.S. Pat. Nos. 5,525,008 and 5,611,642). Also proposed is a method in which sodium hypochlorite and ultraviolet irradiation are used in combination (U.S. Pat. No. 5,582,741). Another method is also known in which a photocatalyst comprised of fine semiconductor particles of an oxide such as titanium oxide and liquid ethylene chloride are suspended under an alkaline condition, and the suspension is irradiated by light to effect decomposition (Japanese Patent Application Laid-Open No. 7-144137).
In addition to the foregoing, methods of photodecomposition by ultraviolet irradiation in a gaseous phase without the use of any oxidizing agent have already been attempted. For example, proposed are a method in which a waste gas containing organohalogenated compounds is subjected to ultraviolet irradiation to convert it into an acidic decomposed gas, followed by washing with an alkali solution to make it harmless (Japanese Patent Application Laid-Open No. 62-191025), and a system in which waste water containing organohalogenated compounds is subjected to aeration and the gas being discharged is subjected to ultraviolet irradiation, followed by washing with an alkali solution (Japanese Patent Application Laid-Open No. 62-191095). It is also known to decompose ethylene chloride using iron powder (Japanese Patent Application Laid-Open No. 8-257570). In this case, it is presumed that reduction decomposition takes place. Reduction decomposition is also reported with respect to the decomposition of tetrachloroethylene (hereinafter abbreviated to xe2x80x9cPCExe2x80x9d) using fine silicon particles.
Chlorinated aliphatic hydrocarbons such as trichloroethylene (hereinafter abbreviated to xe2x80x9cTCExe2x80x9d) and PCE are known to be aerobically or anaerobically decomposed by microorganisms. Attempts have also been made to decompose or purify by utilizing such a process.
An object of the present invention is to provide a decomposition process that does not require any treatment with activated carbon or microorganisms and by which pollutants can be decomposed efficiently, without causing any secondary pollution on account of the use of air, which contains chlorine and may also produce a small quantity, of waste water; and an efficient pollutant decomposition system employing such a process.
To achieve the above object, the present inventors conducted extensive studies. As a result, they have reached a new finding that superior decomposition power can be achieved by subjecting functional water (e.g., acidic water) to aeration to form air that contains chlorine, and mixing this air with air that contains pollutants such as organochlorine compounds, followed by photodecomposition. The functional water is obtained by electrolysis of water that is reported to have a microbicidal effect (Japanese Patent Application Laid-Open No. 1-80293) or the effect of cleaning contaminants present on semiconductor wafers (Japanese Patent Application Laid-Open No. 7-51675).
In the course of continued, detailed experiments to explore any practically desirable form, it was also discovered that in order to conduct a simpler and more efficient decomposition, it is effective to carry out electrolysis on a functional water waste liquor formed in the course of aeration or after the aeration and to form functional water that is again usable as a chlorine feed source. Thus, the water can be reused to carry out the decomposition, making it possible to greatly cut down the quantity of waste water and that of the electrolyte to be added. Accordingly, the present invention has been accomplished.
More specifically, the present invention provides a process for decomposing pollutants by bringing pollutants contained in air into contact with air that contains chlorine under irradiation by light, the process comprising:
a chlorine-containing air generation step of generating air which contains chlorine, by bringing air into contact with a chlorine-generating solution comprised of functional water (I) or functional water (II) having been fed into a chlorine generation region;
a decomposition step of decomposing the pollutants by bringing the air which contains chlorine and air which contains pollutants into contact with each other under irradiation by light in a decomposition treatment region;
a regeneration step of obtaining functional water (II) by regeneration by feeding as functional water waste liquor at least part of the chlorine-generating solution present in the chlorine generation region, to means for forming functional water by electrolysis; and
a feed step of feeding to the chlorine generation region the functional water (II) obtained through the regeneration step;
the functional water (I) and functional water (II) being water capable of generating by aeration the air which contains chlorine, and the functional water (I) comprising a solution used for its formation which does not contain the functional water waste liquor and the functional water (II) comprising a solution used for its formation which contains the functional-water waste liquor at least in part.
The present invention also provides a pollutant decomposition system for decomposing pollutants by bringing pollutants contained in air into contact with air which contains chlorine, under irradiation by light, the system comprising:
a chlorine generation region into which a chlorine-generating solution comprising functional water (I) or functional water (II) is fed to bring it into contact with air to generate air which contains chlorine;
a decomposition treatment region into which the air which contains chlorine and air which contains pollutants are fed to bring them into contact with each other under irradiation by light to decompose the pollutants;
means for effecting irradiation by light;
means for forming functional water by electrolysis; and
means for feeding at least part of the chlorine-generating solution to the means for forming functional water;
the functional water (I) and functional water (II) being water capable of generating by aeration the air which contains chlorine, and the functional water (I) comprising a solution used for its formation which does not contain the functional water waste liquor and the functional water (II) comprising a solution used for its formation which contains the functional water waste liquor at least in part.
The contact of the chlorine-generating solution with the air in the chlorine-containing air generation step may be the step of sending air to the surface of the chlorine-generating solution. In order to improve efficiency, it is preferable to use the step of enlarging the area of gas-liquid contact. To enlarge the area of gas-liquid contact, preferably usable is the step of jetting the chlorine-generating solution into air or subjecting the chlorine-generating solution to aeration.
According to the present invention, a pollutant decomposition process and a pollutant decomposition system used therefor can be provided by which pollutants such as organochlorine compounds can be decomposed efficiently, safely and simply in the gaseous phase under normal temperature and normal pressure, and the quantity of the electrolyte to be added and the quantity of waste water can be cut down.